The aim of the research described in this proposal is to understand the molecular basis of spontaneous mutations caused by transposable elements and the mechanisms whereby mutations at unlinked suppressor loci reverse the phenotype of these spontaneous mutants. We will study mutations at the Drosophila yellow and forked loci, caused by the insertion of the gypsy element, which are suppressed by mutations at the suppressor of Hairy-wing and/or suppressor of forked loci. Preliminary results suggest that the transcriptional activity of the gypsy element is important in causing the mutation and in mediating the phenomenon of suppression. This hypothesis will be tested by analyzing the mutagenic effect of in vitro-altered gypsy elements introduced into the Drosophila germline by P element-mediated transformation; this mutagenic effect will be correlated with the pattern of transcription of the altered element in normal and suppressed flies. In addition, the su(Hw) and su(f) loci will be isolated and characterized. Two different strategies will be used to clone the su(Hw) locus. The first one will involve the use of hybrid dysgenesis to isolate a P-induced allele; the second approach will involve chromosomal walking from a cDNA clone located at 88B1. In order to isolate the su(f) locus we will take advantage of an allele caused by the insertion of the gypsy element. Once the suppressor loci have been isolated the protein products they encode will be identified and their interaction with the gypsy element will be analyzed using in vitro systems to test hypothesis on the molecular mechanisms by which suppression takes place.